The present invention relates to a method for preparing a finely and uniformly mixed carbon-containing composition containing a metal oxide and simple carbon suitable for the manufacture of a ceramic powder for sintering.
More specifically, it relates to a method for preparing a carbon-containing composition containing a metal oxide and simple carbon suitable for the manufacture of a metal carbide, a metal nitride, a metal carbonitride, a metal oxynitride and the like.
As methods for preparing metal carbide powders which are useful as materials of ceramics for sintering, there have heretofore been known
(1) a reduction/carbonization of a metal oxide with carbon,
(2) a direct carbonization of a metal, and
(3) a gaseous phase reaction between a decomposable metal compound and a hydrocarbon.
As methods for preparing a metal nitride powder, there have been known
(4) a method in which a mixture of a metal oxide and carbon is heated up to a high temperature in the atmosphere of a nitrogen-containing compound (e.g., nitrogen or ammonia),
(5) a direct nitrization of a metal, and
(6) a gaseous phase reaction between a decomposable metal compound and a nitrogen compound such as ammonia.
Furthermore, as a method for preparing a metal carbonitride powder, a method is known in which a conversion of a metal oxide is made with carbon in an amount insufficient to convert the metal oxide into a metal carbide but excessive to convert the same into a metal nitride in the above-mentioned method (4).
Moreover, as a method for preparing a metal oxynitride powder,
(7) a method is known in which a conversion of a metal oxide is made with carbon in an amount insufficient to convert the total of the metal oxide into a metal nitride in the above-mentioned method (4).
In the manufacture of a ceramic powder of the metal carbide, metal nitride, metal carbonitride, metal oxynitride or the like by the above-mentioned method, a mixture of a metal oxide and carbon is important as its manufacturing material.
The obtained ceramic powder is usually molded and then sintered. In preparing the sinter, the finer and purer the particles of the ceramic powder are and the sharper a particle diameter distribution is, the higher the density of the sinter is (hereinafter referred to as xe2x80x9ceasy sintering propertiesxe2x80x9d) and the higher the strength of the sinter is.
In order to prepare such a preferable ceramic powder, it is necessary to control the mixing state of a mixture of a metal oxide and carbon which are materials in consideration of a reaction mechanism. Heretofore, the mixing of the metal oxide and carbon has usually been carried out by mechanically mixing a fine metal oxide powder and a carbon powder by the use of a kneader, a mixer or the like.
In this method, however, these powders are mechanically mixed, and so, when mixed, the powders furiously flutter about and working environment is worsen. In addition, there is a problem that impurities are easily entrained owing to the mechanical friction of the kneader, the mixer or the like.
Moreover, the fine metal oxide and carbon having a size of micron order are usually in the state of secondary aggregates, and the finer these aggregates are, the larger their cohesive force is. Therefore, even if the stirring power of the machine is set to a maximum level, these powders cannot be separated into independent particles, so that it is difficult to uniformly mix them. In addition, there is a density difference between the metal oxide and carbon. Therefore, in the case of the mechanical mixing, there is a substantial problem that it is very difficult to obtain a xe2x80x9cuniformxe2x80x9d mixture, though xe2x80x9ca fine statexe2x80x9d can be achieved.
In order to solve these problems, the present inventors have suggested, in Japanese Patent Publication No. 61-30613, a method in which a decomposable metal compound and a decomposable carbon compound are introduced into a hot gas containing water vapor to decompose them, whereby a desired mixture of the metal oxide and carbon is formed in a gaseous phase. According to this method, the uniform mixture can be obtained in which the fine metal oxide and carbon are uniformly mixed. The employment of this mixture permits manufacturing a fine metal carbide having a sharp particle diameter distribution.
However, the decomposable metal compound which is used in this method has some drawbacks in industrial production.
The first drawback is that these decomposable metal compounds are extremely difficult to handle because of being chemically active. For example, the decomposable metal compounds are easily combustible substances having a low flash point, and so, when burned, they generate a toxic gas and when reacted with moisture in the atmosphere, they form the mist of the metal oxide inconveniently. Consequently, in order to safely utilize these substances, additional facilities such as an explosion-proof equipment, a fire-resistant structure and a nitrogen sealing device are required, which increases a manufacturing cost.
The second drawback is that most of these decomposable metal compounds are expensive.
The third drawback is that the relatively inexpensively available decomposable metal compounds usually contain chlorine in the forms of TiCl4 and SiCl4 whose part may be substituted by a hydrocarbon group. Therefore, in the reaction for obtaining a desired carbon-containing composition, hydrochloric acid is formed as a by-product, and in the case of a specific reaction, chlorine is produced inconveniently. In consequence, the mixture of the metal oxide and carbon is contaminated with hydrochloric acid whose amount is trace, with the result that this hydrochloric acid vigorously corrodes devices in the subsequent steps. In addition, corroded substances coming from the corroded devices are entrained as fatal impurities in an easily sinterable ceramic powder which is finally manufactured. In order to prevent these disadvantages, an additional equipment such as a device for removing the corroded components is required, and facilities made of an anti-corrosive high-quality material are necessary in the subsequent steps. Moreover, when a chlorine gas is generated, much attention must be paid to the safety of working circumstances.
As described above, it has been heretofore possible to provide a composition of a metal oxide and carbon suitable for the manufacture of an easily sinterable ceramic powder, but it has been difficult to safely and inexpensively manufacture the composition on an industrial scale.
An object of the present invention is to provide a method for safely and inexpensively preparing a carbon-containing composition having the same fine structure as in a carbon-containing composition described in Japanese Patent Publication No. 61-30613-without carrying out any mechanical mixing operation by the use of a kneader or a mixer which gives rise to the formation of powder dust and the contamination of a composition of a metal oxide and carbon with impurities.
Furthermore, another object of the present invention is to provide a method for preparing a metal carbide powder and a metal nitride powder.
The present inventors have intensively investigated to achieve the above-mentioned objects, and as a result, they have found that a carbon-containing composition in which a fine metal oxide and carbon are uniformly mixed can be obtained by mixing and dispersing a metal oxide and/or a metal compound, which can be converted into the metal oxide by heating, with and in a decomposable carbon compound, and then introducing the resultant dispersion into a hot gas, and that a high-purity and fine easily sinterable ceramic powder having a sharp particle diameter distribution can be obtained by heating the carbon-containing composition. On the basis of these knowledges, the present invention has been completed.
Therefore, the present invention is directed to a method for preparing a carbon-containing composition which comprises the steps of introducing, into a hot gas, a dispersion obtained by dispersing, in a decomposable carbon compound, a metal oxide and/or a metal compound which can be converted into the metal oxide by heating, to form a carbon-containing composition containing simple carbon and the corresponding metal oxide; and then collecting the formed carbon-containing composition.
Furthermore, the present invention is directed to a method for preparing a metal carbide powder which comprises the steps of introducing, into a hot gas, a dispersion obtained by dispersing, in a decomposable carbon compound, a metal oxide and/or a metal compound which can be converted into the metal oxide by heating, to form a carbon-containing composition containing simple carbon and the corresponding metal oxide; and then heating the thus formed carbon-containing composition in an inert atmosphere.
In addition, the present invention is directed to a method for preparing a metal nitride powder which comprises the steps of introducing, into a hot gas, a dispersion obtained by dispersing, in a decomposable carbon compound, a metal oxide and/or a metal compound which can be converted into the metal oxide by heating, to form a carbon-containing composition containing simple carbon and the corresponding metal oxide; and then heating the thus formed carbon-containing composition in a nitrogen-containing atmosphere.